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We present an efficient, bright, and robust source of photons at the rubidium D1-line (795 nm) with a narrow bandwidth of δ = 226(1) MHz. The source is based on non-degenerate, cavity-enhanced spontaneous parametric down-conversion in a monolithic optical parametric oscillator far below threshold. The setup allows for efficient coupling to single mode fibers. A heralding efficiency of η heralded = 45(5) % is achieved, and the uncorrected number of detected photon pairs is 3.8×10 3 cts/(s mW). For pair generation rates up to 0.5 MHz, the source emits heralded single photons with negligible contamination from higher photon number states as evidenced by g(2) c < 0.01. The source is intrinsically stable due to the monolithic configuration. Frequency drifts are on the order of δ /20 per hour even without active stabilization. By applying mechanical strain, the source frequency can be fine-tuned within a range of ≥ 2 GHz.

The discrete-variables (DV) time-frequency (TF) quantum key distribution (QKD) protocol is a BB84-like protocol, which utilizes time and frequency as complementary bases. As orthogonal modulations, pulse position modulation (PPM) and frequency shift keying (FSK) are capable of transmitting several bits per symbol, i.e., per photon. However, unlike traditional binary polarization shift keying, PPM and FSK do not allow perfectly complementary bases. So information is not completely deleted when the wrong-basis filters are applied. Since a general security proof does not yet exist, we numerically assess DV-TF-QKD. We show that the secret key rate increases with a higher number of symbols per basis. Further we identify the optimal pulse relations in the two bases in terms of key rate and resistance against eavesdropping attacks

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